US2459698A - Modulation system - Google Patents

Description

Jan. 18, 1949. c. E. HALLMARK MODULATION SYSTEM Filed Nov. 20, 1944 UTILIZATION CIRCUIT -MODULI'ITING SIGNAL SOURCE INTERMEDIATE FREQUENCY eemzmnon NVEN'I'OR CLYDE El HALLMARK TORNEY Patented Jan. 18, 1949 UNITED STATES o-rrice 2-,4sas9a f I MODULATION sYS -EEM I Clyde. n. Hallmark, Fort mag,managin by mesne assignments, toFarnswonth-Reseanch. Corporation, a. corporation of Indiana ApplicatioirNovem'ber 20, IQQQQSEYEAENQ53%31Q5 4. claims. (c1. 179 -1115). r

' 7 This invention relatesto'signalling systems and particularly to-a'paratus adapted to effect signal modulation ofan. ultra-high frequency carrier wave. I

In order to developsignalemo-dulated energy suitable-for radiation, it is customary either to signal modulate the-output circuit of a: carrier frequency generator or to-signal modulatea power amplifier which is. excited at. the. carrier wave frequency; In eithercasaenergy is derived conventionally from a, direct current power source and-.is'converted into alternating; current energy at the desired carrier wave frequencyby means of the generator or the power amplifier. The magnitude of the energy supplied; from thedirect current source. is in accordance with the amplitude of themodulating signal; Accordingto. con.- ventional practice; the carrier wave generator or the power amplifier employs one'or morevacuum tubes which of necessity havecertain definite limitations with regard to-their current carrying capacities. The current capacity of these tubes determines-the maximum current which may be drawn from the direct-current power source for the.- purpose of. developing power for radiation at the carrier wave frequency. Consequently, for agiven tube complement,- it is possible to increase the magnitude of the radiated energy only by in.- creasing the voltagedeveloped in the power conversion circuit. This, of course, necessitates an increase of'the voltage of the direct current power supply,

'It' generally is undesirable to increase the volt- "age of the direct current power source beyond certain limits. 'sary to employ power sources of inconveniently highvoltages in order to radiate the desired power; Inasmuchas in conventional systems it is-customary to obtain the direct current energy byrectifying' alternating current energy of the proper voltage, there are introduced various losses which limit the overall eificiency of the system. Some-of these losses occur in the form of heat dis-- 'sipated by therectifying and filtering apparatus. When the voltage of the direct current energy necessarily is high in order todevelop therequired power, there are encountered other problems such as those of insulating the rectifier'com patients and the filter elements.

It'isan object of the present invention, thereficre; toprovide apparatus in a signalling system so: arranged as to supply energy to a relatively 'highzir'equencypower conversion circuit directly fronrarelatively low: frequency source of alter:- nating current energy and: thereby obviate the For many purposes, it is neces- .use.-of.- a high voltage directcurrentpower supply.

Another. object of the invention is to impress signakmodulated; intermediatefrequency energy upon apower. conversion circuitofi a radio: ire.- auency: enera-tor as the: primary energy supply for the conversion circuit.

Inaccordance with this invention there is provi'ded aasource. of alternating, current energy having, a relatively low frequency and. relatively high power capacity. In addition, there is provided relatively high frequency means which is supplied with power in substantial quantities from the relatively low frequency alternating current source of energy. By this-means there is; developed alternating current energy'ofrelatively high: power at arelatively' high frequency.

More specifically, there is provided,,in one. form ofthe-invention, anv ultra-high frequency oscillator having an output circuit which: includes a frequency determining component. The apparatusalso;v includes" a power amplifier" whichflis excited. at an intermediate. frequency and is modulated by a signal for theproduction of. intermediate frequency energy modulated. in: amplitude in accordance with the signal. frequencyenergy is coupled. into the output: cirr- Glut of the oscillator so as to supply energy there-- toatiarelative-ly high voltage. A utilization circuit is; also. coupled into. the output circuit of the Referring now to the drawing; there is shown a high frequency generator which may be entirely conventional: consisting for example; of

a tuned-grid tuned-plate oscillator. The oscillator' includes a pair of vacuum tubes H and 2:; botlr of which preferably are high frequency triodes. and may be RCA type 8025; The control grids: of. these tubes: are interconnected by a resonant network 3, comprising the parallelcon:- nection of coil 'd and a condenser 5. This net Work is: tuned for-resonance 'ata relatively high radiofrequency. The cathodes. of the tubes I The intermediate.

' and 2 and the center point of the coil 4 are contion. The other terminal of the coil 9 is connected to ground through a relatively low voltage source of direct current such as a battery 1 which is shunted by a condenser II for by-passing the alternating current present in the circuit. This battery has been found to produce somewhat better operation of the device under some circum! stances, particularly at the higher intermediate frequencies, the development and function of which are tube-described subsequently. It, howeve'rfis contemplated to be within the scope of the" instant invention to eliminate this battery entirely, if desired, which may be done when operating'at somewhat lower intermediate frequencies.

A utilization circuit I2, such as a transmitter for example, is supplied with energy from the high frequency generator by means of a coil l3 co'nnectedto the utilization circuit and coupled inductively to the coil 1 of the resonant network 6 which comprises one ofthe frequency determining circuits of the high frequency generator.

. There also is provided a generator [4 of intermediatefrequency energy which is developed in an output circuit, including a resonant network [5 comprising the parallel connection of a coil 1 Band a condenser 57-. One terminal of the network is connected to one of the output circuit terminals of the intermediate frequency generator and the other network terminal "is connected to an intermediate point of a source of direct current energy such as a battery I8. The negative terminal of this energy source and the other terminal of the generator 14 are connected to' ground. The portion of the battery 18 connected to supply energy to the intermediate frequencygenerator i4 is shunted by a condenser IQ for the purpose or" lay-passing theintermediate frequency energy. around the battery.

The system also includes a power amplifier illustrated herein as one of the push-pulltype consisting of apair of vacuum tubes 20 and 21. These tubes may be tetrodes, as'shown, capable of .relativelyhigh power output. However, other tubes will. serve equally well, one satisfactory typebeing an RCA807. The control grids22 and, 23 respectively of these power amplifier tubes are connected to the terminals of a resonant network 24 which includes a coil 25 andacondenser 26 .connectedin, shunt therewith. This circuit is tuned for. resonance at the intermediate frequency and the coil 25 is inductively coupled to the :ooil l5. of'the network i5, whereby to effect eXcitation-of'the, power amplifier tubes at the intermediate frequency; The screen grids 21 and. 28 of the tubes are connected to theintermediate point on the battery, l8 and the cathodes are connectedto ground. A source of modulaing signal 29 is connectedbetween the cathodes of the power amplifier tubesand a center tap on the coil 25. H

The anodes of the power amplifier tubes 20 and 2| are connected to the respective terminals of another resonant network 30 comprising a 4 coil 3| and a condenser 32 connected in shunt therewith. This resonant circuit also is tuned to the intermediate frequency. Coil 3| is provided with a center tap which is connected to the positive terminal of the battery I8, whereby to supply space current to the amplifier tubes. The coil 3| is inductively coupled to the coil 9 in theoutput circuit of the high frequency generator and, preferably, the turns-ratio between these coils is such as to produce a substantial voltage step-up between the output circuit of the power amplifier and the output circuit of the high frequency generator- Referring now to the operation of the described modulating system, assume that it is desired to radiate energy at a frequency of the order of 300 megacyoles. In this case the high frequency generator including the tubes l and 2 will develop such a frequency and the tuned output circuit 6 will be resonant at this frequency. If the intelligence signal which-is to bemodulated on the high frequency carrier wave is the video information: of a television system, the frequency of the signal derived from the source 29 may be of the order of 4 megacycles. In such a case it is convenient to'excite the power amplifier, including 'thetubes 20 and 2| at a frequency of the order In this manner there is developed in the ampli- 'fier output circuit-30 energy at the intermediate frequency which is modulated in amplitude; in accordancewith the signal. By means of the step-up transformeraction between the coils 3! and -9there is developed in the coil 9 a voltage of a relatively high order of magnitude, the amplitude of which varies in accordance with the modulating 'signal.,'Ihis voltage serves as the primary source of plate voltage for the. high frequency generator tubes l'and'2. Inasmuch as it is preferable to employ tubes of relatively high current ratings for the high frequency generator,

there is effected a' current flow through the coil 1 of the frequency determining circuit 6 of substantial magnitude. Then, by reason of the dc velopment of the relatively high plate voltage for the high frequency generator tubesby the coil 9, ity is possible to develop substantial quantities of power'in the coil 1. The power thus developed jalso'is modulated in accordance'with the modulating signal as a consequence of the voltage variation in the coil 9 being in accordance with the modulating signal. As a result-there is effected a power trrnsferfrom the coil 1 to the coil [3 at the relatively high frequency and of a magnitude varying in accordance with the modulating signal,

It will be obviousto those skilled in the art that a modulating signal such as that chosen for illustration herein, which fundamentally is of an alternating current character,, may be used directlyafter suitable amplification, to supply the required high voltage .to the output circuit of the relativelyhigh frequency generator. In such a case a generator such as the intermediate frequency generator 14 may be eliminated.

Inasmuch as the high frequency generator, including the tubes 1 and 2, functions only during alternate half cycles of the intermediate frequency, it is necessary that this generator be capable of developing power which varies through a range from substantially zero to a maximum value in the time interval of a quarter cycle of the intermediate frequency. It thus will be appreciated that, when operation is at a relatively high intermediate frequency, the build-up time from minimum to maximum power by the high frequency generator is extremely short. In such a case it has been found advantageous to include a relatively low voltage source of direct current energy such as the battery It] in the plate circuit of the generator tubes I and 2. By this means the operation of the generator may be efiected in such a manner that the tubes I and 2 are nonconducting during a smaller portion of the alternate inoperative half cycles of the intermediate frequency voltage developed in the coil 9. In other words, the build-up time for the high frequency generator is lengthened to considerably more than the time interval of a quarter of a cycle of the intermediate frequency voltage. If desired, of course, the voltage of the battery may be selected as substantially one-half of the peak-to-peak voltage of the intermediate frequency energy. In such a case, the high frequency generator will be operative substantially one hundred per cent of the time. Such a condition, however, is not essential for the successful operation of apparatus embodying this invention. Satisfactory results have been obtained by usin a battery 10 of approximately 120 volts when the voltage developed in the coil 9 is several thousand volts. As previously indicated, when the frequency of the voltage developed in the coil 9 is somewhat lower than the 20 megacycle frequency specified herein for illustrative purposes, the battery l0 may be eliminated entirely so that the sole source of plate voltage for the high frequency enerator tubes l and 2 is derived from the coil 9. One such instance would be the case where the modulating signal itself is used to develop the alternating current voltage in the coil 9,

It, therefore, is seen that, by the use of a modulating system according to this invention, it is not necessary to provide apparatus for furnishing direct current energy at prohibitively high voltages. Also, a system of this character is capable of operating so as to effect wide band modulation of ultra-high frequency energy such as required, for example, in a television system.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that Various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. In a signalling system, means for developing intermediate frequency energy modulated in accordance with a signal, power converting means having an output circuit including a resonant network tuned to a radio frequency, means for exciting said network at said radio frequency, means for amplifying the voltage of said intermediate frequency energy, means for supplying said voltage-amplified intermediate frequency energy to said network as the primary source of power for said network, and means for deriving from said network radio frequency energy modu lated in amplitude in accordance with said signalmodulated intermediate frequency.

2. In a modulation system, an amplifier having an output circuit, means for exciting said amplifier at an intermediate frequency, means for modulating said amplifier in accordance with an intelligence signal, a radio frequency generator having an output circuit, means including 10 a voltage step-up transformer for coupling said radio frequency generator output circuit to the output circuit of said amplifier, the primary winding of said transformer being connected in the output circuit of said amplifier and the secondary winding of said transformer being connected in said radio frequency generator output circuit, and a utilization circuit coupled to said radio frequency generator output circuit.

3. In a modulation system, a power amplifier having an input circuit and an output circuit,

means including an intermediate frequency generator coupled to said input circuit for exciting said amplifier, means including a source of modulating signal coupled to said amplifier to effect the development in said output circiut of intermediate frequency energy modulated in amplitude in accordance with said signal, a radio frequency generator having an output circuit tuned to said radio frequency, means including a coil for coupling said radio frequency generator output circuit to the output circuit of said amplifier, said coil being connected in said radio frequency generator output circuit and forming the secondary winding of :a voltage step-up transformer of which the primary winding is included in the output circuit of said power amplifier, and a utilization circuit coupled to said radio frequency generator output circuit.

4. In a modulation system, a push-pull power ,3) amplifier comprising two vacuum tubes each having an input circuit and an output circuit, means including an intermediate frequency oscillator coupled to said input circuits for exciting said amplifier, means including a source of modulating signal coupled to said amplifier to effect the development in said output circuits of intermediate frequency energy modulated in amplitude in accordance with said signal, a radio frequency oscillator having an output circuit tuned to said radio frequency, a source of constant magnitude direct current energy connected in said tuned output circuit, means including a voltage step-up transformer for coupling said radio frequency oscillator output circuit to the output circuits of said amplifier tubes, and a utilization circuit coupled to said tuned oscillator output circuit.

- CLYDE E. HALLMARK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number

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